Flame-made ternary Pd-In2O3-ZrO2 catalyst with enhanced oxygen vacancy generation for CO2 hydrogenation to methanol

Palladium promotion and deposition on monoclinic zirconia are effective strategies to boost the performance of bulk In2O3 in CO2-to-methanol and could unlock superior reactivity if well integrated into a single catalytic system. However, harnessing synergic effects of the individual components is crucial and very challenging as it requires precise control over their assembly. Herein, we present ternary Pd-In2O3-ZrO2 catalysts prepared by flame spray pyrolysis (FSP) with remarkable methanol productivity and improved metal utilization, surpassing their binary counterparts. Unlike established impregnation and co-precipitation methods, FSP produces materials combining low-nuclearity palladium species associated with In2O3 monolayers highly dispersed on the ZrO2 carrier, whose surface partially transforms from a tetragonal into a monoclinic-like structure upon reaction. A pioneering protocol developed to quantify oxygen vacancies using in situ electron paramagnetic resonance spectroscopy reveals their enhanced generation because of this unique catalyst architecture, thereby rationalizing its high and sustained methanol productivity.

(1) Up to now, the conversion of CO2 is still less than 10% in single pass, far below industrial criterion. Since the activity, or to say, STY, is too low. With this respect, the way the research team working on is in right track. However, the selectivity declined mildly, below 90%.
(2) According to results SI Figure 1, no matter how the components are, with or without Pd, ZrO2, the selectivity to methanol remained unchanged. It is well known that the structure InOx change little during the reaction. This paper gave less information for the elucidating this phenomenon.
(3) As shown in SI Figure 1,the high activity for FSP Pd-In-Zr has been attributed to more oxygen vacancy creating on ZrO2 surface. If comparing the performance of sample 8 (CP, Pd-In) and sample 10 (FSP, Pd-In-Zr), they showed a very close performance, ZrO2 seems contributes little to the improvement of catalysts. Thus, all studies became groundless. (4) Another reason for this high activity of FSP-Pd-In-Zr is assumed to quick H2 dissociation, it may not be true, because H2 dissociation is very fast in the surface with abundant of oxygen vacancy. (5) The mechanism for CO2 activation on the surface of those catalysts may be worth to study in the further.
Reviewer #2 (Remarks to the Author): In this manuscript, the authors have investigated a flame spray pyrolysis (FSP) method to prepare ternary Pd-In2O3-ZrO2 catalyst prepared in one step manner. The catalyst prepared via FSP route shows high methanol productivity from hydrogenation of CO2 and the observed activity is attributed to an enriched density of oxygen vacancies on the surface of catalyst and high dispersity of active components. Generally speaking, the catalysts derived from different preparative routes (FSP, CP & WI) were studied with comprehensive analytical characterization. I must say that the work was executed with care and the paper presentation format is excellent. The detailed investigation would then be very useful the researchers in the field of carbon dioxide utilization. Thus, the paper can be recommended for its publication in this Journal after the following revisions: (a) MeOH productivity (such as the statement on page 4: "which reaches a record productivity of 1.3 gMeOH h-1gcat-1") also depends on reactant flow rate adopted, or gas hourly space velocity (GHSV). Therefore, a comparison table for the current work and literature reports under the same GHSV should be presented in Supplementary Information. (b) Estimation of the thickness of surface In2O3 layer should be done for both FSP and WI catalyst samples.
(c) While single-atom In is detected in Supplementary Figure 10b, how to confirm they are indeed In adatoms instead of Pd adatoms?
(d) A comparison of surface texture (i.e., surface area and porosity) and morphology of different particles prepared by different methods (FSP, CP and WI types). This piece of information is important as it determines the catalyst performance in general. Three catalysts must have similar (if not identical) surface area and porosity in order to have a fair comparison on other parameters that dictate the catalytic performance.
(e) Similar structural analysis (i.e., Supplementary Figure 10) should be done for those prepared by WI method in order to demonstrate the catalyst model described in Figure 9 (WI-derived catalyst), that is, forming the surface sheets of In2O3.

Manuscript NCOMMS-22-21949-T -Response to Reviewers
Notes: Comments in blue -Replies in black -Actions in bold Indicated page, line, figure, or reference numbers refer to the revised manuscript and/or supporting information with changes highlighted

Reviewer #1
Unique architecture of ternary Pd-In2O3-ZrO2 catalyst prepared by flame spray pyrolysis unlocks high performance in CO2 hydrogenation to methanol. This work reported the Pd-In2O3-ZrO2 catalyst prepared by a FSP method, which showed the high activity to the hydrogenation of CO2 to methanol, being 3-5 times higher than those catalysts from traditional approaches. The structure of catalysts has been characterized using multi techniques, such as TEM, XRD and in situ EPR etc. However, after reviewing the advances in the presented catalyst, my impression is that the work did not make a significant progress in this topic. The following suggestions may help for consideration.
We thank the Reviewer for assessing our study. Still, we regret that they did not appreciate the significance of our contribution, which is well recognized by Reviewer #2. We believe that our contribution brings unique progress to the field of CO2 hydrogenation because: • Despite being considered a pivotal strategy for realizing a prospective industrial implementation of In2O3-based catalysts, no work thus far has attempted to integrate palladium, zirconia, and indium oxide into a ternary Pd-In2O3/ZrO2 system. This task is indeed very challenging as the effect of simply combining the three components does not necessarily translate into a linear performance enhancement. • We have not only pioneered the development of ternary Pd-In2O3/ZrO2 catalysts but, more importantly, identified flame spray pyrolysis (FSP) as a key synthesis method to effectively produce materials with a unique architecture, which, as pointed out by the Reviewer, display up to 3-5 times higher activity than that of counterpart catalysts prepared by traditional synthetic approaches, such as co-precipitation and wet impregnation. • We reshape the understanding of the effect of ZrO2 polymorphs on In2O3-catalyzed methanol synthesis, showcasing that a pure monoclinic phase is not critical to unlocking high performing catalytic systems. • Last but not least, we address the long-standing challenge of characterizing and quantifying oxygen vacancies on In2O3-based catalysts, thereby advancing understanding of this critical performance descriptor and offering a new approach that can be potentially extended to assess the density of oxygen vacancies on other relevant reducible oxides acting as heterogeneous catalysts for diverse applications. To better highlight these key aspects, which may not have been recognized in our original contribution, we have amended the title and rewritten the abstract, introduction, and conclusions.
1. Up to now, the conversion of CO2 is still less than 10% in single pass, far below industrial criterion. Since the activity, or to say, STY, is too low. With this respect, the way the research team working on is in right track. However, the selectivity declined mildly, below 90%.
We strongly disagree with the validity of this comment. CO2 hydrogenation to methanol is an equilibrium limited transformation. As such, thermodynamics dictates that the maximum single-pass CO2 conversion (XCO2) reachable by an ideal catalyst would be ca. 30% under the reaction conditions applied in our study (T = 553 K, P = 5 MPa, and H2/CO2 = 4). [1] Additionally, it is important to emphasize that even at the same temperature, pressure, and H2/CO2 ratio, XCO2 is also significantly impacted by the gas-hourly space velocity (GHSV), with its absolute value decreasing as GHSV increases. [2] Therefore, based on the aforementioned discussion and considering the high GHSV used in our study to investigate catalysts in the kinetic regime and thus assess their intrinsic performance (48,000 cm 3 h −1 gcat −1 , ca. 2-fold higher than that generally applied in other studies), the XCO2 of our Pd-In2O3/ZrO2,FSP catalyst, which is actually ca.12%, is certainly not far from what is expected from industrial standards. To further clarify this point, we have added the Supplementary Table 3 comparing XCO2, methanol selectivity (SMeOH), and methanol space-time yield (STY) at similar reaction conditions of various relevant catalysts in CO2 hydrogenation to methanol. As shown in Supplementary Table 3, the STY of our ternary system is 1-2 orders of magnitude higher than existing systems in relation to the indium content, strengthening the potential implementation of this process. In addition, since SMeOH highly depends on the XCO2 levels at which it is measured, with the former generally decreasing as the latter increases, [3] attaining a SMeOH of ca. 90% at a high XCO2 level such as 12% is truly outstanding and confirms the intrinsic high selectivity of our catalyst.
[3] Chem. Soc. Rev., 2020, 49, 1385-1413 2. According to results SI Figure 1, no matter how the components are, with or without Pd, ZrO2, the selectivity to methanol remained unchanged. It is well known that the structure InOx change little during the reaction. This paper gave less information for the elucidating this phenomenon.
SMeOH data displayed in Supplementary Figure 1 was measured at distinct XCO2 levels, therefore, it cannot be unequivocally used to compare SMeOH among different catalysts. However, Supplementary  Figure 2 depicts SMeOH measured at the same XCO2 level for all catalysts prepared by FSP, which in contrast to the Reviewers' claim, confirms that the presence of palladium and ZrO2 improves SMeOH when compared to that of bulk In2O3. Similar observations were also reported for binary Pd-In2O3 and In2O3/m-ZrO2 systems synthesized by co-precipitation and wet-impregnation, respectively. [1] Concerning the In2O3 structure, it is well-known that it can significantly change upon reaction, depending on the synthesis method, the type of carrier and metal promoter, and reaction conditions used. [2] In fact, we show an example in our study in which In2O3 sinters into large particles upon reaction when supported on tetragonal zirconia by wet impregnation, whereas it remains well-dispersed as monolayer-like platelets when deposited on the corresponding monoclinic polymorph (Figure 4a). Additionally, there have also been reports showing that over-reduction of bulk In2O3 into inactive metallic indium can take place during reaction owing to the presence of carbon monoxide generated as byproduct and metal promoters.  Figure 1, the high activity for FSP Pd-In-Zr has been attributed to more oxygen vacancy creating on ZrO2 surface. If comparing the performance of sample 8 (CP, Pd-In) and sample 10 (FSP, Pd-In-Zr), they showed a very close performance, ZrO2 seems contributes little to the improvement of catalysts. Thus, all studies became groundless.

As shown in SI
As mentioned in the response to Comment #2, SMeOH data displayed in Supplementary Figure 1 was measured at distinct XCO2 levels, therefore, it cannot be unequivocally used to compare SMeOH among different catalysts. We attributed the high performance of Pd-In2O3/ZrO2,FSP catalysts to augmented density of oxygen vacancies created on In2O3, which is fostered by both Pd and ZrO2, as uncovered by electron paramagnetic resonance (EPR) spectroscopy (Figure 7 and 8). Additionally, while the Reviewer claims that no significant difference in performance exists between the Pd-In2O3,CP and Pd-In2O3/ZrO2,FSP catalysts, if we compare their methanol productivity (see STY in Figure 2 and Supplementary Table 3), we observe that methanol productivity over the ternary catalyst is 25% and 97% superior to that of Pd-In2O3,CP, on the basis of mass of catalyst and indium, respectively. Therefore, it seems clear that ZrO2 greatly contributes to improve performance, particularly because it does so while requiring a significantly lower content of indium, which alike palladium is also a scarce and expensive material. This point has been further highlighted on page 9, lines 188-190.

Another reason for this high activity of FSP-Pd-In-Zr is assumed to quick H2 dissociation, it may not be true, because H2 dissociation is very fast in the surface with abundant of oxygen vacancy.
Thank you for raising this important point. Indeed, H2 dissociation is considered virtually barrierless on Pd-In2O3/ZrO2,FSP, as suggested by measurements of apparent reaction order in H2 (Figure 6b). Still, this most likely does not occur exclusively because of an abundant density of oxygen vacancies, but rather due to a high concentration of active ensembles comprising both isolated palladium atoms and oxygen vacancies, as previously reported for metal promoted-In2O3 catalysts. [1] In fact, it is welldocumented that H2 activation is energetically demanding and limits methanol synthesis over unpromoted In2O3, since oxygen vacancies split H2 in a heterolytic manner while simultaneously activating CO2. [2] For this reason, deposition on a monoclinic zirconia carrier and introducing a palladium promoter have been used as efficient strategies to improve the performance of bulk In2O3. [3] While both palladium and m-ZrO2 improve H2 splitting by creating additional vacancies on In2O3, the former further contributes to a greater extent to this process, since palladium itself activates H2 in a homolytic manner, which is barrierless compared to heterolytic H2 activation on oxygen vacancies and suggested to increase the abundance of surface H* species that promote the hydrogenation steps. Overall, we agree with the Reviewer and hypothesize that the high performance of Pd-In2O3/ZrO2,FSP most likely stems from its ability to facilitate specific C-H hydrogenation steps rather than H2 dissociation itself. We have amended this discussion in the revised manuscript on page 15, lines 335-337.

The mechanism for CO2 activation on the surface of those catalysts may be worth to study in the further.
We thank the Reviewer for this valuable suggestion. Indeed, shedding light on the mechanism for CO2 activation on ternary Pd-In2O3/ZrO2 systems is worth studying further. Still, it would require a detailed experimental investigation, comparing both ternary systems and their binary counterparts through operando methods using isotopically labeled compounds, and therefore deserves a dedicated study. We have added a sentence to the amended manuscript pointing to the relevance of such a study in the future (page 15, lines 341-343).

Reviewer #2
In this manuscript, the authors have investigated a flame spray pyrolysis (FSP) method to prepare ternary Pd-In2O3-ZrO2 catalyst prepared in one step manner. The catalyst prepared via FSP route shows high methanol productivity from hydrogenation of CO2 and the observed activity is attributed to an enriched density of oxygen vacancies on the surface of catalyst and high dispersity of active components. Generally speaking, the catalysts derived from different preparative routes (FSP, CP & WI) were studied with comprehensive analytical characterization. I must say that the work was executed with care and the paper presentation format is excellent. The detailed investigation would then be very useful the researchers in the field of carbon dioxide utilization. Thus, the paper can be recommended for its publication in this Journal after the following revisions: We are delighted to read the Reviewer's positive feedback regarding the impact of our study. We have carefully acted upon all their suggestions, which were highly valuable.
1. MeOH productivity (such as the statement on page 4: "which reaches a record productivity of 1.3 gMeOH h − gcat −1 ") also depends on reactant flow rate adopted, or gas hourly space velocity (GHSV). Therefore, a comparison table for the current work and literature reports under the same GHSV should be presented in Supplementary Information.
We agree with the Reviewer on this very relevant point. Accordingly, we have added Supplementary  Table 3 comparing XCO2, SMeOH, and STY at similar reaction conditions of various relevant catalysts in CO2 hydrogenation to methanol, which is included in the response to Comment #1 of Reviewer #1. Additionally, we have also replaced the word "record" by "outstanding" as the former can have an ambiguous meaning.

Estimation of the thickness of surface In2O3 layer should be done for both FSP and WI catalyst samples.
Based on the aberration-corrected scanning transmission electron microscopy (AC-STEM) analysis (e.g., as presented in Supplementary Figures 11 and 13), the In2O3 phase in the catalyst prepared by both flame spray pyrolysis and wet impregnation appears to wet the surface strongly, leading to a typical thickness of no more than a monolayer thick. This observation agrees with the high and uniform dispersion of indium over the carrier particles evidenced in elemental maps of the catalyst acquired by energy dispersive X-ray (EDX) spectroscopy (Figure 4d). It is also consistent with structures we previously observed for In2O3 supported on monoclinic zirconia including single and groups of indium oxide adatoms. [1] We have discussed the thickness of the In2O3 layers in the caption of Supplementary Figure 11. [1] ACS Catal. 2020, 10, 1133. Supplementary Figure 10b, how to confirm they are indeed In adatoms instead of Pd adatoms?

While single-atom In is detected in
The Reviewer correctly points out that that it is not possible to differentiate between palladium and indium atoms based solely on AC-STEM images. Our assignment takes into account several other factors i) EDX maps indicate a higher dispersion of In on the ZrO2 carrier than of Pd (Figure 4d), ii) as mentioned above isolated adatoms of indium have been previously observed for binary systems comprising In2O3 supported on m-ZrO2, [1] and iii) palladium species supported on ZrO2 by flame spray pyrolysis show a greater tendency to form nanoparticles upon reaction than when the metal is supported on In2O3, confirming the less preferential stabilization on ZrO2 (Supplementary Figure 12). We have elaborated on the discussion of the assignment of In adatoms in the caption of Supplementary  Figure 11. [1] ACS Catal. 2020, 10, 1133.

A comparison of surface texture (i.e., surface area and porosity) and morphology of different particles prepared by different methods (FSP, CP and WI types). This piece of information is important as it determines the catalyst performance in general. Three catalysts must have similar (if not identical) surface area and porosity in order to have a fair comparison on other parameters that dictate the catalytic performance.
We agree with the Reviewer and thank them for raising this relevant point. Accordingly, we have measured the surface area and pore volume of both binary In2O3-ZrO2 and ternary Pd-In2O3/ZrO2 catalysts prepared by co-precipitation (CP), wet impregnation (WI), and flame spray pyrolysis (FSP), which are summarized in Supplementary Figure 9. The results show that all systems possess similar textural properties, except for Pd-In2O3-ZrO2, CP. The latter displays an inferior surface area compared to its counterparts, further confirming that CP is not an ideal synthesis method to produce Pd-In2O3-ZrO2 catalysts with superior performance. Hence, this sample was considered unsuitable for a fair comparison with the other ternary systems. Figure 10) should be done for those prepared by WI method in order to demonstrate the catalyst model described in Figure 9 (WI-derived catalyst), that is, forming the surface sheets of In2O3.

Similar structural analysis (i.e., Supplementary
A similar structural analysis was performed for the Pd-In2O3/ZrO2 catalyst prepared by WI by microscopy techniques and has been added to Supplementary Figure 13 upon revision. The results confirm the formation of monolayer-like In2O3 platelets on the ZrO2 surface.